This invention relates to diesel engines for locomotives and the like; and, more particularly, to diesel engines whose emissions must meet Tier 2 emissions standards promulgated by the Environmental Protection Agency (EPA).
In a diesel engine, fuel is directly injected into a cylinder of compressed air at a high temperature. The fuel is broken up into droplets which evaporate and mix with the air forming a combustible mixture. Products of combustion of this mixture are exhaust emissions that include hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM). To reduce the amount of pollution in the atmosphere, the EPA regulates the emission level of these various exhaust products that is acceptable. Over time, the acceptable levels of emissions have changed. For locomotive diesel engines, the newest standards for emissions for locomotives manufactured on or after Jan. 1, 2005 are referred to as Tier 2 standards. These standards were published in the Federal Register, Vol. 63, No. 73 on Thursday, Apr. 16, 1998. The Tier 2 emission limits (in grams/horsepower/hour) for a line haul locomotive are as follows:
hydrocarbons (NC)0.30carbon monoxide (CO)1.5oxides of nitrogen (NO3)5.5particulate matter (PM)0.20steady state smoke (ss)2030-second transient smoke403 second transient smoke50.
Attainment of these standards involves consideration of a number of factors relating to engine operation. These include such things as injection pressure and injection timing, nozzle spray patterns, hydraulic flow, manifold air temperature, compression ratio, and air/fuel ratios. As will be appreciated by those skilled in the art, changes to effect reduction of one type of emission may well result in an increase in another emission component. For example, retarding fuel injection timing, which effectively reduces NOx, also effects engine performance and increases CO and PM. Other attempts to reduce PM result in an increase in fuel consumption which is also undesirable. In co-pending, co-assigned United States patent application (serial number), there is described a method of operating a locomotive diesel engine so as to meet the Tier 2 NOx requirements.
It is desirable, therefore, to effect a strategy for in-cylinder combustion which satisfies the Tier 2 requirements for PM, in addition to those for NOx, while at the same time maintaining an acceptable level of engine performance including fuel consumption. Importantly, the strategy for meeting Tier 2 requirements for PM, while continuing to satisfy the Tier 2 NOx requirements, should be done without the need for after treatment of the combustion products such as by scrubbing, or otherwise treating the exhaust. Such techniques add cost to the engine, take up space that may not be readily available in confined areas where the engine is installed, and adds to the maintenance requirements for the engine. Rather it is important to achieve the Tier 2 levels solely by controlling engine operation.